1. Field of the Invention
The present invention relates to a method and apparatus for producing curved glass sheets, which also may be toughened, notably for the production of automobile safety panes. More particularly, the present invention relates to methods in which the glass sheets are heated while flat in a straight-through horizontal furnace, are brought by a conveyor into a curving station, where they are stopped beneath an upper element, are raised and applied against this upper element, and are then released onto a lower element, for example of the frame type open at its center, the shaping of the glass sheets being carried out at least partly on this lower element, and then finally are brought into a cooling station, notably a thermal toughening station.
2. Discussion of the Background
Many examples of curving techniques are found in the technical literature. Notably, Patent Publications EP-A-3,391, FR-B-2,085,464, EP-A-240,4l8, EP-B 241,355, U.S. Pat. No. 4,682,997, all have in common the use of an upper element taking part in the transfer of the glass sheets from the conveyor which has brought the glass sheets through the furnace to a lower element which constitutes a shaping tool and/or a tool for transferring the glass sheets to the cooling station. Generally, the contact surface of the upper element with the glass sheet is covered with an intervening covering. This intervening covering softens the contact between glass and upper element, notably by a "smoothing" of the slight imperfections in the surface condition of the upper element which may subsist even after careful machining. Furthermore, this covering absorbs the dust particles sandwiched between the upper element and the glass sheet, which otherwise would leave their imprint on the relatively soft glass. Finally, this covering prevents pitting from forming on the hot glass, due to the difference in the nature of the materials constituting the upper element (for example, refractory steel) and the sheets treated (by definition, glass).
Numerous materials have been used as covering materials. Those most commonly used are fabrics of glass fibers or papers based upon silica and/or aluminum fibers agglomerated by a mineral binder. However, disadvantageously, these coverings must be replaced very frequently--especially if the glass panes comprise many enamelled zones, which cause the glass sheets to stick to the covering material, which results in the tearing away of the covering material in strips as the enamelled glass sheets are released from the upper element.
In another connection, "long-life" coverings have quite recently been proposed, based essentially on metallic fibres. In French Patent Application FR-A-2,606,398, a covering is disclosed in which the surface which contacts the glass is made of a mesh of knitted metallic wires. On the other hand, European Patent Application EP-A-31,439 discloses a basically metallic fabric possessing a thermal conductivity which preferably is less than 0.2 W m.sup.-1 K.sup.-1, formed of rovings of a plurality of elementary threads, disposed in different directions, together creating links or stitches and interfering to form knots at the tops of the stitches, as a result of which the stitches are indeformable.
These last-mentioned coverings wear less rapidly, even if they are brought into contact with enamelled panes having a tendency to stick, this sticking tendency being essentially linked with the nature itself of the enamelling compositions. However, independently of the materials used, the authors of the present invention have established that enamelled panes have an inferior optical quality as compared to that of panes that are identical in shape and size, but not enamelled, simply because of application against a rigid upper element.
To overcome this problem, a curving method is disclosed in European Patent Applications EP-A-240,4l8 and EP-A-241,355, according to which the glass sheet is raised and applied against the upper element by a suction resulting from a pressure reduction created in the vicinity of the periphery of the glass sheet, which projects beyond the upper element, because the face of the upper element, toward the conveyor against which the glass sheet is being applied, is advantageously of smaller dimensions than those of the applied face of the glass sheet. It is thus possible to arrange for the enamelled parts of the pane--which generally are along the edges so as to form a kind of frame--not to be applied against the upper element. Nevertheless, not all the difficulties are entirely eliminated in this way.
In fact, along the demarcation line separating the part of the glass sheet applied against the upper element from its non-applied part, the presence of deformations can be detected which result in optical defects which appear after the toughening. The applied areas do not behave in exactly the same way as the non-applied areas, although a full explanation of the mechanisms to which these differences in behavior can be attributed has not yet been found.
Thus, there remains a need for a method and apparatus for preparing curved glass sheets, which are free of the above-described drawbacks.